Identification and differential expression of multiple isoforms of mouse Coiled-coil-DIX1 (Ccd1), a positive regulator of Wnt signaling

The Wnt signaling plays important roles in cell growth, differentiation, polarity formation, and neural development. In the canonical pathway, two DIX domain-containing proteins, Dishevelled (Dvl) and Axin, regulate the degradation of β-catenin that activates Wnt target genes through TCF/LEF family transcription factors. Recently, we have isolated a third type of DIX domain-possessing protein, Coiled-coil-DIX1 (Ccd1). Ccd1 forms homomeric and heteromeric complexes with Dvl and Axin, and regulates the neural patterning in zebrafish embryos through Wnt pathway activation. Here, we report the isolation and characterization of mouse Ccd1. Fourteen putative mRNA isoforms are generated by different promoter usage and alternative splicing, and each isoform shows different expression patterns in various tissues. The predicted Ccd1 proteins are classified into three subtypes, and a novel form, termed Ccd1A, possesses an N-terminal calponin homology domain, suggesting an additional interaction of the isoform with actin or other proteins. When Ccd1 proteins were singularly expressed in Hela cells, they showed almost no activation of TCF-dependent reporter transcription on their own. However, when Dvl protein, at the level that did not activate Wnt pathway by itself, was co-expressed with Ccd1, the reporter transcription was greatly potentiated in Ccd1-dose-dependent manner. In addition, Ccd1- and Wnt3a-dependent activation of Wnt pathway was inhibited by Axin or a dominant negative Ccd1. These results indicate that mouse Ccd1 functions as a positive regulator of the Wnt/β-catenin pathway. Furthermore, Ccd1 is highly expressed and co-localized with Wnt signaling molecules in the embryonic and adult brain, implicating the importance of Ccd1 in the Wnt-mediated neuronal development, plasticity, and remodeling.